This invention is directed to a control device for a forced air heating system which includes a heat probe responsive to the temperature of the discharged air of the furnace wherein the heat probe is connected to an optical coupled switch means which controls the gas flow to the furnace burner. More particularly, the heat probe includes an integrated circuit capable of sensing a high temperature and a low temperature whereby the fuel flow to the burner is shut off in response to the high temperature and is turned on in response to the low temperature.
For generations forced air furnaces were constructed to operate in essentially two modes. These were either off when no heat was needed or on when there was a demand for heat. In the on position the burner burned at its maximum rate at all times. In the days of readily available and cheap energy sources little consideration was given to the inefficiencies of these types of furnaces.
In more recent times with a enlightened awareness of finite fuel resources, steps have been taken to improve the efficiencies of forced air furnaces. While more modern and efficient furnaces are available for the consumer, because of the expense of these furnaces the consumer is not apt to replace the existing furnace unless the existing furnace becomes totally inoperable. The consumer presently then is caught between the high replacement costs of their existing furnace and the increasing costs of fuel to operate the same. As fuel costs escalate more and more, the pay back period for replacing an inefficient furnace will decrease, however, presently even in the face of increasing fuel bills the pay back time for replacing furnaces is still too long to warrant replacement of the same.
It has been recognized, as for instance, by the disclosures of U.S. Pat. Nos. 3,126,154; 3,921,899; 4,240,579; and 4,423,765 that modifications of the operation cycle of the existing furnaces can be made to make them more efficient. All of the above patents share in common the recognition that modifications can be made to existing furnaces to increase the efficiencies of these furnaces. However, before any of these devices will be widely accepted and therefore widely used by the consumer, the devices must be capable of ease of installation, adaptability to a variety of furnaces having different characteristics and capable of a long, useful service life equivalent to that of the furnace itself.
Certain of the devices described in the above mentioned patents utilize bi-metal strips and/or mechanical relays in their construction. Both of these require the use of contact points for completion of an electrical circuit. Unfortunately, bi-metal strips and mechanical relays which utilize contact points are subject to catastrophic failure. This is very evident by the inclusion in U.S. Pat. No. 4,240,579 of a breakage notch in its bi-metal strip and a flexible connector electrically connected on either side of the breakage notch which will allow for operation of the furnace in a normal manner, i.e., in the manner it operated before the adaption of the device to the furnace in case the bi-metal strip fails and breaks. Disclosed in this same patent is a method for sensing the temperature of the exhaust flue with modification of the burner operation made in response to the exhaust flue temperature.
U.S. Pat. No. 3,126,154 also utilizes bi-metal strips. Since the bi-metal strips of this patent are not located in the high temperature exhaust flue they are less prone to fail. However, the control device of this patent is, in effect, responsive to room temperature and not to direct operation of the furnace.
U.S. Pat. No. 4,423,765 does not rely on mechanical relays or the like for electrical switching control. It utilizes certain solid state devices. However, this patent as well as U.S. Pat. No. 3,921,899 relies on timing circuits to control the operation of the burner. In both of these the burner is turned off in response to sensing of a pre-determined temperature. However, the burner is not turned on in response to heat needs of the furnace, but instead it is turned back on in response to a pre-set time period. Because the weather patterns in a particular area may change from day to day and certainly change from season to season optimum cycle time for maximum efficiency of the burner coupled with adequate heat delivery also varies. Devices, therefore, which rely on burner operation dependent upon cycling times require constant monitoring and readjustment depending upon seasonal and short term weather variations and their corresponding variability on heat demand from the furnace.